My theory is (and it may be completely wrong) that; the more water there is, the less space is left for steam, so it takes longer to build up enough steam force to push the water up, and meanwhile the water gets hotter.

Interesting ways to put it. There’s something there, but I’m getting the impression it’s a trickier mechanical engineering problem than this.Is there someone here who has a mech. eng. background? Ken understands these things much more than I do.Water quantity is an important factor and it sounds like it’s nonlinear. With the other factors (heat, amount of grounds, atmospheric pressure…), it may all create something that requires either precise knowledge of what goes on.

In my case, it’s really just about experimenting and tweaking. After using moka pots for so long, my understanding of how they really work remains very limited. But I usually find a way to produce the coffee I want. The inconsistencies in the final product are part of the fun.

My theory is (and it may be completely wrong) that; the more water there is, the less space is left for steam, so it takes longer to build up enough steam force to push the water up, and meanwhile the water gets hotter.

Anyway, what you say is true but I'd be shocked to see any serious increase in temp (just thinking of the affects on T in PV=nrT) a slight increase in V wouldn't seem to yield much increase in T. But I'd have to see something more scientific to actually know. There could be other variables in play.

My theory is (and it may be completely wrong) that; the more water there is, the less space is left for steam, so it takes longer to build up enough steam force to push the water up, and meanwhile the water gets hotter.

I don't think this is it. If anything the steam should build pressure slightly faster with less space to fill.

There are two sources of pressure. There is the steam, the pressure of which is related to the water temperature. But there is also trapped air in the head space above the water. This is initially at a pressure of nearly 1 atmosphere. As the water heats up this air partial pressure should rise along with the steam partial pressure.

But I wonder: Under this pressure, is water forced up into the grounds, even as the valve is blocking the exit at the top? If this is the case then the head space volume will increase.

While the steam pressure depends only on the temperature, the air partial pressure will drop if the air volume increases. The amount it drops depends on the relative volume change, which in turn depends on the initial volume, i.e., how high the pot is filled with water. With less air pressure the water has to be hotter to achieve the same total pressure.

I've played around with this idea in a mock-up of a regular moka pot and found that water level has a significant effect on temperature. But the Brikka is a different beast.

Okay. I don't have a Brikka (yet). But with a regular moka pot it makes sense that it should happen. I've seen it with mine.

But the final temperature is also affected by grind, coffee type, heating rate, initial water temperature, and atmospheric pressure. So it can be possible to change one thing and then to compensate by changing something else.

I did a bunch of this a while back using a glass jar to simulate a moka pot. There was definately an effect due to different water levels (more precisely, different head space volumes). But with an actual moka pot I don't have many data (yet). I started down that path a while ago and then got distracted with another aspect of brewing, became frustrated with that, and then set the whole thing aside and went back to simply brewing and enjoying coffee. :o)

Anyways, here is a single side by side comparison of two moka runs with different amounts of water. With my pot 176g is just below the valve and 225g is just above the valve. I had thermistors embedded in the pot and a means for determining flow through the grounds. It's not exactly the picture I was expecting, but I see quite a bit of variability from brew to brew. So take this with a grain of salt.

So the brewing actually starts at 45C. Didn't expect the grounds to get wet so early, interesting.

And according to the graph it seems important not to wait till the end of the brewing process, and pour the coffee a bit earlier (which i've been doing), to avoid a burnt taste.

But as i said, with the electric stove i now have; if the water level is a bit low, then the water doesn't even get hot enough for a proper brew -- i'm just guessing, but it could be ending up around 85C or so. It didn't used to be this way with the gas stove, so the heating profile is definitely important. But if one can not fine-tune the heat level, one can fine-tune the water level to get the brew right -- which is good to know.

These were just two brews, in a specific moka pot (not likely yours), brewed with a certain procedure. I wasn't trying to make good coffee so much as have a consistent means for comparing the results of varying different parameters. And I succeeded in that I made a number of really lousy cups of coffee! But my point is that you shouldn't draw too many conclusions from this graph.

Oddly enough these two brews didn't taste burnt to me. The larger volume one had some bitter notes, and the weight of the oven dried grounds suggested a pretty high extraction. All else equal, more water results in a higher extraction. I think this is a general rule, at least for percolation brew methods.

To embed the probes I took out the safety valve and ran small wires through that port.

A final water temperature of 85°C sounds pretty low. It's possible if the geometry is right, you have really loose grounds, and you heat really slowly. Do you live at high altitude? Maybe you're right, but I'll bet it was hotter than you think.

One last picture, this one of a different brew showing water temperature and grounds temperature versus brew time. It's obvious when the grounds were first infused with water.

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